Sunday, December 21, 2008

Organic Reaction Mechanisms

An organic reaction occurs between a substratum (an organic compound) and an attacking agent termed a reagent.

1. Bond Cleavage (Homolytic or Heterolytic Cleavage)

The bonds in the organic compound break and reaction intermediates are formed. These reactions intermediates are:

1. Free radicals
2. Carbocations (An ion that has carbon atom with positive charge)
3. Carbanion (An ion with carbon atom with negative charge)
4. Carbene (singlet and triplet)
5. Radical ion

Homolytic Fission

Free radicals form as intermediates

Heterolytic Fission

Carbocations, carbanions and carbenes form during this type of fission

2. Reagents

Electrophiles, Nucleophiles and Ambiphiles are reagents.

This reagent requires electrons.
Examples: ions like NO2+, Cl+
and atoms in molecules like S03 and BF3.


They are negatively charged and attack positively charged atoms.
Examples ions like OH-, CN-, H- or
atoms in molecules like N in NH3 and O in H2O.


H2O It has an electron attacking and electron repelling site also.

3. Inductive Effect:
The study of this effect is important because of its implication for the stability of reaction intermediates.

When an electron-withdrawing or electron-releasing group (substituent) is attached to carbon chain, polarity is induced on the carbon atom and on the substituent attached to it. This is called inductive effect or simply as I-effect.

Negative Inductive Effect (-I Effect)
Postive Inductive Effect (+I Effect)

4.Electromeric effect
In presence of an electrophile, there is complete transfer of π electrons from one atom to other to produce temporary polarity on atoms joined by mutliple bonds. It is called electromeric effect.

Positive Electromeric effect
Negative Electromeric effect

5. Mesomeric effect

In conjugate systems (compounds having alternate π and sigma bonds) π-electron shifting takes place consecutively giving permanent polarity on the chain. This π-electron shift in conjugate systems is called mesomeric effect or conjugate effect.


6. Hyperconjugation

When an alkyl group is attached to a double bond (unsaturated system in general), the hydrogen in the alkyl group releases an electron and becomes positive. And in the process there is no bond formation between C and H+. This type of electron release is called hyperconjugation.

7. Types of Reactions


7a. Addition Reactions
This reaction is characteristic of the compounds having double or triple bonds.

7b. Elimination Reactions

In most elimination reactions, two groups on adjacent atoms are lost and a double bond is formed. Thus, the product of an eliminatin reaction is an alkene.

7c. Substitution Reactions

In this reaction, one atom or group of atoms in a molecule is replaced by another.

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